Chlor-alkali systems and processes produce chlorine, sodium hydroxide (caustic soda) and other caustic alkali products. Typically, the process is conducted in an electrolytic membrane cell using a brine, which is an aqueous solution of sodium chloride. The brine is fed into the electrolytic cell, which includes an anode side and a cathode side separated by a membrane. A current is passed through the electrolytic cell. As a result, the sodium chloride brine splits into its constituent parts. The membrane allows sodium ions to pass through it to the cathode side, where it forms sodium hydroxide in a solution. The membrane allows only positive sodium ions to pass through to prevent the chlorine from mixing with the sodium hydroxide. The chloride ions are oxidized to chlorine gas at the anode. Hydrogen gas and hydroxide ions are formed at the cathode. After this process, the brine is depleted and cannot be used in the electrolytic cell. Therefore, the depleted brine must be treated or replaced with fresh brine in order for the membrane electrolytic cell to properly operate. Further, the depleted brine must be purified to remove impurities that may cause fouling of the membrane.
The prior art has attempted to address the need for purified brine with limited success. For example, U.S. Pat. No. 4,169,773 to Lai, et al. discloses a system and method for the electrolytic production of alkali metal hydroxide and halide with acidification of part of a recirculating anolyte stream to remove halite. The system in Lai diverts a brine stream from a membrane cell to a reaction vessel for treatment. In the reaction vessel, concentrated hydrochloric acid (HCl) is added to the brine stream to minimize chlorine dioxide production. The treated stream is then irradiated with ultra violet light. The irradiated stream is passed through a scrubber before being reintroduced to the membrane cell. However, the system in Lai requires the use of hydrochloric acid and an irradiation step that increases costs and inefficiencies.
U.S. Pat. No. 6,309,530 to Rutherford, et al. discloses a system and method for the concentration of depleted brine exiting a chlor-alkali membrane cell plant. The depleted brine flows from a membrane cell into a dechlorinator where chemicals, such as sodium carbonate and sodium hydroxide are added. The dechlorinated brine is fed into a concentrator system where water vapor is removed. The reconcentrated brine is then ready for use. However, like Lai, the system in Rutherford requires the addition of chemicals, thereby leading increased costs.
Therefore, there is a need in art for a system and method for purifying brine that does not add chemicals to the brine and does not require costly compression and/or condensation steps. Thus, there is a need for a system and method of purifying depleted brine with minimal costs and steps to treat the depleted brine.